In German Offlegungsschrift No. 2442260 is disclosed an electric tool in which the deformation of the handle relative to the housing is measured for monitoring and, optionally, for limiting the torque and, thereby, reducing the danger of accidents at the increasingly higher power and torque of such tools. The drive motor is controlled in dependence on the measured value. The tool uses, for obtaining the measurement value, strain gauges or piezo resistors which are preferably arranged in a region of reduced housing cross section. Determining the torque in this way has, for one, the disadvantage that strain gauge and piezoelectrically operating sensors provide only very small measurement values, the processing of which is complicated and subject to interference. Measuring the stress of the housing in the region of the handle means, in addition, that it is necessary to adapt the measuring pick-up, together with the circuit, to each housing design. In addition, there is the danger that the monitoring will not work if the tool, under special circumstances or due to negligence, is not gripped by the handle, or if accessories reduce or prevent the stress on the handle.
The present invention is concerned with solving these problems.
Pressure sensitive conductive plastic material which is also called "conductive plastic", is known in many variants. For instance, German Auslegungsschrift No. 25 43 455 discloses the use of silicone rubber mixed with metal particles or carbon dust for such material. The use of conductive plastic material as a pressure sensitive switch for overload protection is disclosed in German Patent No. 20 45 385. And, the use of such material as a continuously variable resistor for controlling the current drain of consuming devices, for instance, for controlling the speed of hand drills, is disclosed in U.S. Pat. No. 3,386,067.